1. Field
The aspects of the present disclosure generally relate to extrusion die systems. In particular, the aspects of the present disclosure relate to the cyclical extrusion of materials to generate small sized grain features, generally in the range of micro and nanosized grain features to improve the strength of extruded products
2. Brief Description of Related Developments
Nanostructured materials are generally regarded as materials having very small grain feature size, typically in the range of approximately 1-100 nanometers (10−9 meters). Metals, ceramics, polymeric and composite materials may be processed in a variety of ways to form nanosized features. These materials have the potential for wide ranging applications, including for example, industrial, biomedical and electronic applications. As a result, a great deal of study is ongoing to gain a better understanding of the characteristics of these materials.
Conventional extrusion formed products are limited to approximately twelve layers. Micro-layer extrusion processes can extend these limitations. A micro-layer extrusion process that provides a method for obtaining small grain features is described in U.S. Pat. No. 7,690,908, (the “'908 Patent”) commonly owned by the assignee of the instant application, the disclosure of which is incorporated herein by reference in its entirety. The disclosure of the '908 Patent describes a system of dies that are constructed to adapt extrusion technology. Examples of such extrusion technology are described in U.S. Pat. Nos. 6,669,458, 6,533,565 and 6,945,764, commonly owned by the assignee of the instant application.
The typical micro-layer product is formed in a sheet. If a tubular product is desired, the microlayer is first formed into a sheet and then made into the tube. This creates a weld line or separation between the microlayers. The '908 Patent describes a cyclical extrusion of materials by dividing, overlapping and laminating layers of flowing material, multiplying the flow and further dividing, overlapping and laminating the material flow to generate small grain features and improve properties of the formed product. Examples of the improved properties include, but are not limited to burst strength, tensile strength, tear resistance, barrier and optical properties. Referring to FIGS. 1 and 2, a series of die plates 100 are configured to receive a flow of material, such as plastic or other suitable material, at 101. The first distribution module 110 divides the flow into multiple capillary streams and distributes the flow downstream to a transition module 111. The transition module 111 further divides the streams and transforms the shape of the generally circular capillary streams into thin ribbon like streams. At the exit of the transition module 111 the flow cross sectional area of each ribbon stream may be reduced by a compression of the flowing material. At the output of the transition module 111, sets of adjacent ribbons (a pair as shown) are directed to a first compression stage in die plate 6 of compression/layering module 112. Each set of ribbon pairs is processed into a layered flow. This layered flow comprises a laminate of the sets of adjacent ribbons. The first stage compression die plate 6 also splits the laminated ribbons into at least a pair of adjacent ribbons. At this stage, the original plastic flow is considerably altered and now comprises at least side by side, multiple parallel flows in the form of laminated ribbons.
As shown in FIGS. 1 and 2, each of these flows are subjected to a series of cycles in which the ribbons are compressed, divided and overlapped to multiply the number of laminations. In one embodiment, this cycle is repeated in a chain of extrusion stages of this construction, resulting in increasing numbers of thinner laminations formed within the extrusion flow. In the case of dual side by side flows, the number of laminations would be doubled at each stage.
By first distributing the flowing plastic into a set of multiple streams and then combining the multiple streams into a series of laminated streams, a group of parallel streams, oriented above and below, as shown in FIG. 1, may be processed in parallel and rejoined to generate combined flow stream. At the die system exit, generally shown at 106, a laminated plastic flow is achieved having a significantly high number of thin laminations in which micro and nano-sized features may be formed. By controlling the number of compression, division, and lamination cycles, the thickness of the laminations may be adjusted. As described in the '908 Patent, it is advantageous that a basic die system include dividing the stream into at least a pair of flow streams to take full advantage of multiple cycles of compression, dividing and layering. In the '908 Patent, multiple pairs of flow streams are stacked in a parallel relationship.
The '908 Patent relies on the cyclical extrusion of materials. The output plastic flow, also referred to as the rejoined flow, is applied to final die elements 120 that wind the laminated plastic flow into a tubular end product having nano-sized features. U.S. Pat. No. 6,669,458, (“the '458 Patent”), commonly owned by the assignee of the instant application and the disclosure of which is incorporated herein by reference in its entirety, discloses one embodiment of an extrusion die with rotating components. Referring to FIG. 3, a cross-sectional view of an extrusion die having balanced flow passages and rotating elements is illustrated.
The extrusion system 150, shown in FIG. 3 is configured to extrude a tubular product constructed of common thermoplastic materials. The system 150 includes an extruder 152 designed to provide a molten material, such as plastic, to an extrusion die 153. The extrusion die 153 consists of a series of components including a die body 154, and a die module 156. When assembled, the extrusion die 153 of these components is constructed having a passage 157 therein extending from an upstream inlet 158 to a downstream outlet 159. The passage 157 is formed by the cooperation of adjacent components and the individual components of the passage communicate to provide a continuous passage 157 for the flow of molten plastic through the extrusion die 153. This passage is constructed to provide a balanced flow of plastic to and throughout an extrusion channel 23 which is formed downstream as described below.
Flow channel(s) 161 are connected to inlet 158 and a divider 162 separates the incoming stream of plastic evenly into the two channels. Flow channels 161 are constructed in the die body 154 and extend through the die body 154 to respective outlets (not shown), in the transversely oriented downstream face 165 of the die body 154. A distribution groove 166 is formed in the downstream face 165 between an upstream edge and a downstream edge. The distribution groove 166 communicates with the outlets to receive molten plastic from the flow channels 151. The distribution groove 166 is substantially semicircular in cross section and extends in an annular manner concentric with the axis 24 of the extrusion die 3. The flow of plastic will be around the distribution groove 166 from each of the outlets. The flow will be in two opposing paths within the groove 166.
The die body 154 and die module 156 are constructed with axially extending bores 25 and 26 respectively which align to form a continuous opening along the axis 24 of the extrusion die 153. A tip module 155 is constructed to fit within the bore 25/26. A clearance is formed between the inner surface of the bore 26, and the outer surface of the tip 155 to form the extrusion channel portion 23 and the exit portion 27 of the plastic passage 157. A conical surface 22 is constructed on the outer surface of the tip module 155 and cooperates with a conical portion of the bore 26 to form the tapered extrusion channel 23. The tip 155 may be constructed with an axial bore 30 to allow an elongated element to pass through the die for coating. The '458 Patent provides for relative rotational movement between the surfaces 28 and 29.
A rotary die assembly requires specialized parts and configurations to accommodate the rotating surfaces as well as the high temperatures involved in these processes. It would be advantageous to be able create a tubular product using an extrusion system with or without the need for rotating components.
All tubular products made by an extrusion die possess a knit or weld line due to the supports required for the tip or mandrel in the dies center. A rotary die twists the polymer, so the knit lines get mixed into a seamless product, potentially eliminating the weld line and making the tube stronger. The tube also spins when it exits the die, which can create a helical appearance i.e. addition of stripes. The spinning tube can also be utilized to aid in a coating process.
Accordingly, it would be desirable to provide a system that addresses at least some of the problems identified above.